2018MNRAS.478..487N

H I observations of the Local Group dwarf irregular galaxies Sextans A and B, obtained with the Karoo Array Telescope (KAT-7) are presented. The KAT-7 wide field of view and excellent surface brightness sensitivity allows us to verify the true H I extent of the galaxies. We derive H I extends of 30 and 20 arcmin and total H I fluxes of 181 ± 2.0 and 105 ± 1.4 Jy km s^–1 for Sextans A and B, respectively. This result shows clearly the overestimate of the H I extent and total flux of 54 arcmin and 264 Jy km s^–1 reported for Sextans A using the Effelsberg observations. Tilted ring models allow us to derive the rotation curves (RCs) of Sextans A and B out to 550 arcsec (∼3.5 kpc) and 650 arcsec (∼4 kpc), respectively. The RCs of the two galaxies are seen to decline in the outer parts. The dark matter distribution in Sextans A is better described by the pseudo-isothermal halo model when an M/L ratio of 0.2 is used. For Sextans B, the mass model fits are not as good but again an isothermal sphere with an M/L of 0.2 represents best the data. Using the MOdified Newtonian Dynamics, better fits are obtained when the constant a₀ is allowed to vary. The critical densities for gravitational instabilities are calculated using the Toomre-Q and cloud-growth based on shear criterion. We find that in regions of star formation, the cloud growth criterion based on shear explains better the star formation in both Sextans A and B.